2 Answers
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In the studio era, most of the film crew worked at the same studio for years and years. Only the most prominent members of the crew got screen credit.

Since the mid-1960s, just about everybody on a film crew (excepting animation) tends to free-lance. So getting on-screen credit is very important to helping someone get another film-related job. (As Paulie_D mentioned above, craft unions have demanded screen credit for the many unheralded people that work on films.) Thus we now see credits for drivers, caterers, lawyers and accountants on films. In old movies, even those with "a cast of thousands", stunt men (or women) were rarely credited. In today's action movies, dozens of stunt people get on-screen credit

And since movies are much more expensive, there are many more people working on them. Special effects crews can be huge now.

Studios used to own their own music libraries. But now they have to pay to use certain songs or recordings. So all of these songs are listed in the credits. If the movie contains original songs or music, the composers and performers certainly deserve credit. Plus listing this information helps people purchase soundtrack recordings.

Hitchcock actually filmed PSYCHO (1960) on a small budget using his crew that worked on his television series. So even if they listed everyone involved in the film, the credits would still be short by today's standards.

It takes a lot of people make a movie, and everybody deserves credit for it. It would be like graduating from high school. But only the names of the top 20 students get to be in the yearbooks. That's the way it used to be. You would work on a movie, and pour your heart and soul into it. And you would get no credit for it. Some people even put their lives and safety on the line. And they too, would get zero acknowledgement. As time went on, people started to realize that this was unfair. They eventually got the courage to unionize, stand up to the studios, and demanded to be credited.